1. Field of the Invention
The present invention relates to an image forming apparatus such as a color laser printer, a color copying machine, or a color facsimile machine that mainly employs an electrophotographic process, and more particularly to alignment control of each color developer image formed on an image bearing member.
2. Description of the Related Art
Conventionally, in a color image forming apparatus including a plurality of photosensitive drums, positional deviation occurs between color images due to mechanical mounting error of the photosensitive drum, an optical path length error of each laser beam, or an optical path change. Thus, a method has been proposed to correct the positional deviation between the color images, in which a positional deviation correction pattern is produced on an intermediate transfer belt and the amount of position deviation (hereinafter referred to as “a positional deviation amount”) between the color images is corrected by detecting the position of the positional deviation correction pattern.
Japanese Patent Application Laid-Open No. 2009-93155 discusses a detection method that uses a sensor for detecting the positional deviation correction pattern using a diffused light reflected from the deviation correction pattern.
When the positional deviation correction pattern is detected using diffused reflected light, an output value of diffused reflected light from a black developer formed on the intermediate transfer belt is reduced almost equally to that of the diffused reflection light from the intermediate transfer belt. Accordingly, as illustrated in FIGS. 15A and 15B, the positional deviation correction pattern is a pattern formed by using a pattern of a color developer as a base and superposing a pattern of black developer on the color developer.
In the examples shown in FIGS. 15A and 15B, black developer patterns 1604 are respectively superposed on a yellow developer pattern 1601, a magenta developer pattern 1602, and a cyan developer pattern 1603. Accordingly, the black developer pattern having limited diffused reflection light can be detected.
Japanese Patent Application Laid-Open No. 2007-272111 discusses a density increase that occurs at an image trailing edge during image formation in an electrophotographic process. Hereinafter, the occurrence of a density increase at the image trailing edge will be referred to as “sweeping”.
Referring to FIG. 16, the mechanism by which sweeping occurs at the image trailing edge will be described. The amount of developed developer increases at a boundary on the downstream side of a latent image region 1703 on a photosensitive drum 1701. That is, at the boundary on the downstream side of a latent image region 1703, developer 1704 stuck to a developing roller 1702 facing the latent image region 1703 and a region on the downstream side of the facing position flies to the latent image region 1703 side with a low potential.
Thus, the amount of developer flying to the rotational-direction downstream side of the photosensitive drum 1701 is larger than that of developer in the latent image region 1703 other than the boundary on the downstream side, causing a sweeping phenomenon where the density of at the image trailing edge increases as illustrated in FIG. 17.
To reduce the sweeping amount, Japanese Patent Application Laid-Open No. 2007-272111 discusses a technique for extracting contour information from image information and setting, based on the extracted contour information, an image density of a region where sweeping is supposed to occur to be lower than that of original image data.
Such sweeping also occurs when a positional deviation correction pattern is formed on the intermediate transfer belt. When the sensor detects the positional deviation correction pattern where sweeping has occurred, the trailing edge of the positional deviation correction pattern cannot be correctly detected. Thus sweeping can cause errors in the detection of positional deviation.
For example, when a sensor configured to detect the positional deviation amount by using diffused reflected light is used, as illustrated in FIG. 18A, a positional deviation correction pattern is formed where a black developer pattern 1902 is superposed on a color development pattern 1901 as a base.
In this case, as illustrated in FIG. 18B, because of the influence of sweeping occurring at a pattern trailing edge, the deposition amount of developer at the pattern trailing edge increases to form a highly dense portion. Accordingly, as illustrated in FIG. 18C, in an analog output signal from a detection sensor, indicating a measure of the intensity of the diffused reflected light, the intensity of the trailing edge of a color developer pattern is higher than other positions on the pattern.
In the case of the black developer pattern, similar sweeping occurs at a pattern trailing edge. However, light is absorbed by the black developer itself reducing the amount of diffused reflected light from the developer. Thus, in an output value when the black developer pattern is detected by the sensor, the influence of sweeping is reduced.
The analog output signal from the sensor is binarized with a predetermined threshold value, and a positional deviation amount is calculated based on the timings of a rising edge and a falling edge of the binarized digital output signal. Specifically, as illustrated in FIG. 18D, the center position of the color developer pattern 1901 is calculated based on the timing ty 11 of the detection of the rising edge and the timing ty12 of the detection of the falling edge of the digital output signal.
Similarly, the center position of the black developer pattern is calculated based on the timing tk11 of the detection of the rising edge and the timing tk12 of the detection of the falling edge of the digital output signal. Then, a difference “Δ dy” between the center position of the color developer pattern and the center position of the black developer pattern is calculated as a relative positional deviation amount between the color developer pattern and the black developer pattern.
When no sweeping occurs, the analog signal output from the sensor and the digital output signal binarized with the threshold value are as indicated by broken lines illustrated in FIGS. 18C and 18D. Thus, for example, when there is no positional deviation between the color developer pattern and the black developer pattern, as illustrated in FIG. 18E, a positional deviation amount Δ dy is “0” (Δ dy=0).
However, when sweeping occurs, a highly dense portion is formed at the pattern trailing edge due to the sweeping. Thus, the analog signal output from the sensor and the digital output signal binarized with the threshold value are as indicated by solid lines illustrated in FIGS. 18C and 18D.
Thus, for example, even when there is no positional deviation between the color developer pattern and the black developer pattern, as illustrated in FIG. 18F, a positional deviation amount Δ dy′ is not “0” (i.e. Δ dy′≠0), and erroneously detected as a positional deviation amount.
For example, the influence of the sweeping may be reduced by changing the setting of the image density as in the case of the technique discussed in Japanese Patent Application Laid-Open No. 2007-272111. However, this takes time and labor because one must predict how much sweeping occurs and a positional deviation detection pattern must be formed where the density is lowered accordingly.
When the influence of sweeping is not sufficiently reduced by prediction, the positional deviation amount Δ dy′ is not “0” (Δ dy′≠0), and erroneously detected as a positional deviation amount.
Thus, when sweeping occurs during the formation of the conventional positional deviation correction pattern, an error occurs in the positional deviation amounts between the respective color images of the positional deviation correction patterns formed on the intermediate transfer belt. As a result, there is a problem in that when positional deviation is corrected, based on the output result of the sensor, correction accuracy is reduced due to the influence of the sweeping.